The present invention relates to machines for converting between mechanical and electrical energy, and in particular to a compact high power alternator using permanent magnets suitable for automotive use.
An alternator typically comprises a rotor mounted on a rotating shaft and disposed concentrically relative to a stationary stator. Alternatively, a stationary rotor may be positioned concentrically within a rotating stator. An external energy source, such as a motor or turbine, commonly drives the rotating element, directly or through an intermediate system such as a pulley belt. Both the stator and the rotor have a series of poles. Either the rotor or the stator generates a magnetic field, which interacts with windings on the poles of the other structure. As the magnetic field intercepts the windings, an electrical current is generated, which is provided to a suitable load. The induced current is typically applied to a bridge rectifier, sometimes regulated, and provided as an output. In some instances, the regulated output signal is applied to an inverter to provide an AC output.
Conversely, the device can act as a motor if an appropriate electrical signal is applied to the windings.
Conventionally, alternators employed in automotive applications typically comprise: a housing, mounted on the exterior of an engine; a stator having 3-phase windings housed in the housing, a belt-driven claw-pole type (e.g., Lundell) rotor rotatably supported in the housing within the stator. However, the power output of such conventional claw-pole type alternators is directly proportional to the size of the alternator; to increase power output the size of the conventional alternator must be significantly increased. Accordingly, space constraints in vehicles tend to make such alternators difficult to use in high output, e.g., 5 kW, applications, such as for powering air conditioning, refrigeration, or communications apparatus. In addition, claw-type generators are also disadvantageous in that voltage regulation is by modulating the rotating field. Such modulation affects all of the windings. Accordingly, voltage regulation and control of individual windings is impractical.
In addition, the claw-pole type rotors, carrying windings, are relatively heavy (often comprising as much as half of the total weight of the alternator) and create substantial inertia. Such inertia, in effect, presents a load on the engine each time the engine is accelerated. This tends to decrease the efficiency of the engine, causing additional fuel consumption. Reductions in the mass and diameter of rotating components of an alternator will tend to reduce the overall inertia an engine has to overcome, thereby improving fuel economy. A permanent magnet alternator is ideally suited for reducing overall inertia. The mass and diameter of rotating components are reduced as compared to that of conventional Lundell alternators, while supplying an equivalent amount of power.
A reduction of inertia in a motor vehicle alternator also translates to a reduction in horsepower required by the engine to accelerate the alternator. The savings in horsepower could then conceivably be applied to a vehicle drive train resulting in more power to propel the vehicle. This would be of great interest for example, to race car engineers who must deal with regulations limiting horsepower generated by race engines. Even a slight improvement in available horse power to the drive wheels can yield a tremendous competitive advantage.
In addition, such inertia can be problematical in applications such as electrical or hybrid vehicles. Hybrid vehicles utilize a gasoline engine to propel the vehicle at speeds above a predetermined threshold, e.g. 30 kph (typically corresponding to a range of RPM where the gasoline engine is most efficient). Similarly, in a so-called “mild hybrid,” a starter-generator is employed to provide an initial burst of propulsion when the driver depresses the accelerator pedal, facilitating shutting off the vehicle engine when the vehicle is stopped in traffic to save fuel and cut down on emissions. Such mild hybrid systems typically contemplate use of a high-voltage (e.g. 42 volts) electrical system. The alternator in such systems must be capable of recharging the battery to sufficient levels to drive the starter-generator to provide the initial burst of propulsion between successive stops, particularly in stop and go traffic. Thus, a relatively high power, low inertia alternator is needed.
In general, there is in need for additional electrical power for powering control and drive systems, air conditioning and appliances in vehicles. This is particularly true of vehicles for recreational, industrial transport applications such as refrigeration, construction applications, and military applications.
For example, there is a trend in the automotive industry to employ intelligent electrical, rather than mechanical or hydraulic control and drive systems to decrease the power load on the vehicle engine and increased fuel economy. Such systems may be employed, for example, in connection with steering servos (which typically are active only a steering correction is required), shock absorbers (using feedback to adjust the stiffness of the shock absorbers to road and speed conditions), air conditioning (operating the compressor at the minimum speed required to maintain constant temperature). The use of such electrical control and drive systems tends to increase the demand on the electrical power system of the vehicle.
Similarly, it is desirable that mobile refrigeration systems be electrically driven. For example, efficiency can be increased by driving the refrigeration system at variable speeds (independently of the vehicle engine rpm). In addition, with electrically driven systems the hoses connecting the various components, e.g. the compressor (on the engine), condenser (disposed to be exposed to air), and evaporation unit (located in the cold compartment), can be replaced by an electrically driven hermetically sealed system analogous to a home refrigerator or air-conditioner. Accordingly, it is desirable that a vehicle electrical power system in such application be capable of providing the requisite power levels for an electrically driven unit.
There is also a particular need for a “remove and replace” high power alternator to retrofit existing vehicles. Typically only a limited amount of space is provided within the engine compartment of the vehicle to accommodate the alternator. Unless a replacement alternator fits within that available space, installation is, if possible, significantly complicated, typically requiring removal of major components such as radiators, bumpers, etc. and installation of extra brackets, belts and hardware. Accordingly, it is desirable that a replacement alternator fit within the original space provided, and interface with the original hardware.
In general, permanent magnet alternators are well-known. Such alternators use permanent magnets to generate the requisite magnetic field. Permanent magnet generators tend to be much lighter and smaller than traditional wound field generators. Examples of permanent magnet alternators are described in U.S. Pat. No. 5,625,276 issued to Scott et al on Apr. 29, 1997; U.S. Pat. No. 5,705,917 issued to Scott et al on Jan. 6, 1998; U.S. Pat. No. 5,886,504 issued to Scott et al on Mar. 23, 1999; U.S. Pat. No. 5,929,611 issued to Scott et al on Jul. 27 1999; U.S. Pat. No. 6,034,511 issued to Scott et al on Mar. 7, 2000; and U.S. Pat. No. 6,441,522 issued to Scott on Aug. 27, 2002.
Particularly light and compact permanent magnet alternators can be implemented by employing an “external” permanent magnet rotor and an “internal” stator. The rotor comprises a hollow cylindrical casing with high-energy permanent magnets disposed on the interior surface of the cylinder. The stator is disposed concentrically within the rotor casing. Rotation of the rotor about the stator causes magnetic flux from the rotor magnets to interact with and induce current in the stator windings. An example of such an alternator is described in, for example, the aforementioned U.S. Pat. No. 5,705,917 issued to Scott et al on Jan. 6, 1998 and U.S. Pat. No. 5,929,611 issued to Scott et al on Jul. 27, 1999.
The stator in such permanent magnet alternators is suitably comprised of individual thin steel laminations of an appropriate shape and chemical composition which are then welded or epoxied together in a cylindrical body with teeth and slots to accept windings. The respective laminations of the stack are positioned in both axial and rotational alignment so that the resultant state or teeth and slots are aligned (disposed) axially. The power output wave produced by axially aligned teeth and slots is by its nature a square wave.
However, it would be advantageous in applications employing control systems dependant on synchronization with the output, to have a power output wave with sloping sides to enhance control timing.
The power supplied by a permanent magnet generator varies significantly according to the speed of the rotor. In many applications, changes in the rotor speed are common due to, for example, engine speed variations in an automobile, or changes in load characteristics. Accordingly, an electronic control system is typically employed. An example of a permanent magnet alternator and control systems therefor is described in the aforementioned U.S. Pat. No. 5,625,276 issued to Scott et al on Apr. 29, 1997. Examples of other control systems are described in U.S. Pat. No. 6,018,200 issued to Anderson, et al. on Jan. 25, 2000.
However, in such permanent magnet alternators, the efficiency is inversely proportional to the “air gap” separating the magnets from the stator. Such air gaps are often in the range of 20 to 40 thousands of an inch. With such close spacing/tolerances, the permanent magnet alternators are particularly susceptible to destructive interference (clashing) between magnets and stator as a result of displacement of the rotor caused by external forces acting on the alternator. In vehicular applications relatively severe external forces are commonplace, due to, for example, engine vibration (particularly diesel engines at startup), cornering, traversing bumpy roads or terrain, and other types of impact. Accordingly, an alternator in which rotor displacement is minimized, and which includes a mechanism to absorb unacceptable rotor displacement and prevent the rotor magnets from clashing with the stator is needed.
The use of a taper at the end of a motor shaft to center an attachment, e.g. attaching lawn mower blades to a motor shaft, is known. Conventionally, such a taper is provided only at the end of a shaft. An axial tapped hole is provided in the shaft end surface. The attachment includes a hub with a corresponding tapered aperture. However, the tapered aperture typically extends only partway (as opposed to through) the attachment hub; it is, in effect, a countersink to a smaller diameter through bore. The attachment is secured to the shaft by a bolt passing through the attachment hub bore and threaded into the hole in the shaft end surface. The tapered connection tends to center the attachment on the shaft, however, the attachment on the end of the shaft, is, in effect, cantilevered and susceptible to vibrational oscillations.
In addition, the heat generated by compact high power alternators can also be problematical. This is particularly true in applications where significant levels of power are generated at relatively low engine rpm; in general; the amount of air moved by a fan is proportional to the square of the fan rpm. As alternators become more compact and more efficient, significant levels of heat are generated. Permanent magnets are particularly susceptible to damage due to overheating; under high load, high temperature conditions, such magnets can become demagnetized. Similarly, the electronic components employed in the controller are susceptible to heat damage. Accordingly, a strategy must be developed to dissipate heat buildup.
Use of airflow to cool heat generating elements (e.g., rectifiers) in a gen-set are known. An example of such cooling is described in the aforementioned U.S. Pat. No. 5,929,611 issued to Scott et al on Jul. 27 1999. Conventionally, airflow is provided by a fan driven by the same shaft on which the rotor is mounted. However, in various automotive applications, significant heat is generated at low rpm.
In general, an appreciable reduction in diameters would be employed to achieve a useful reduction in inertia. This tends to create an acute need for cooling in reduced inertia alternators. The reduction in both mass and overall diameters of these alternators tends to make the use of conventional cooling methods impractical.
Cooling techniques that permit a permanent alternator to be fully sealed are desirable in situations where exposure to the elements would be detrimental to the operation of the alternator. This is of particular interest to the military or any application subjected to harsh, dusty environments which would be detrimental to the magnets due to their affinity to ferrous particles found in most sand.
There also is a need for an alternator that can accommodate not only the power levels, but also the space and ruggedness constraints imposed by use in vehicles. For example, operation of a vehicle tends to generate forces perpendicular to the axis of the rotor that are sometimes sufficient to cause the rotor and stator to clash. The rotor and stator are separated only by a small air gap, and the external forces tend to cause transverse movement of the rotor in excess of the air gap then there will be striking interference.